A method of preparing ortho esters

Abstract: Ortho esters of the structure wherein R₁ and R₂ are different or the same alkyl moieties and can be a saturated, unsaturated, branched, unbranched or cyclic alkyl moiety of 1-3 carbon atoms, are prepared by first anhydrously reacting hydrogen cyanide or a hydrogen cyanide derivative with a secondary alkanol and hydrogen chloride in the presence of an inert organic solvent. Upon the formation of the resulting imino ether hydrochloride, more secondary alkanol is added to form the ortho esters (I). 1,1-Dialkoxycycloalkanes of the structure wherein R₁ and R₂ correspond to R₁ and R₂ of structure (I), and n is an integer from 4 to 9, are prepared by reacting with the imino ether hydrochloride more secondary alkanol and a cycloalkanone having 5 to 10 carbon atoms.

Description: The present invention relates to a "one-pot" preparation of ortho esters. The ortho esters are compounds of the structure (I) wherein R 1 and R 2 are different or the same alkyl moieties and can be a saturated, unsaturated, branched, unbranched or cyclic alkyl moiety of 1-3 carbon atoms. The ortho esters defined by structure (I) are useful water scavengers in organic reaction and solvents. The ortho esters can be used to generate an anhydrous medium for reactions or other processes where the presence of water can be a detriment. The compounds defined by structure (I) are also useful intermediates in the manufacture of pharmaceutical, photographic or agricultural products. In the process of the invention, compounds of the structure (I) are prepared by first preparing an imino ether hydrochloride by reacting hydrogen cyanide with a secondary alkanol and hydrogen chloride in the presence of an inert organic solvent. Compounds of structure (I) are then prepared by further addition of secondary alkanol to the reaction mixture. It is known (from S. Patai, Chemistry of Functional Groups, Chemistry of Amidines and Amidates, Wiley, 1975, p. 389-394) that when an organic acid nitrile is reacted with a primary alcohol and hydrogen chloride an imino ether hydrochloride is formed. This reaction scheme, generally known as the "Pinner" reaction, involves two steps for the preparation of ortho esters. First, an organic acid nitrile, primary alcohol and hydrogen chloride are reacted to prepare an imino ether hydrochloride. Second, the imino ether hydrochloride is separated from the reaction mixture by crystallization, washed for purification and subsequently subjected to alcoholysis with additional primary alcohol to afford the ortho ester. Thus, the Pinner reaction is discontinuous by virtue of the purification step and expensive because of the large amount of solvent required. Without the purification step, however, the ortho ester is not formed to a significant extent. Further, in this process, the yield of the ortho ester obtained fluctuates. The ortho ester of the primary alcohol thus obtained can be reacted with a secondary alcohol in the presence of an acid catalyst to obtain the ortho ester of structure (I). In the method of the invention, however, the preparation of orthoesters of structure (I) is continuous because the purification and transesterification are removed. The ortho ester obtained consistently exceeds 70% in yield and 99% in purity. It is known from US-A-2567927 to produce aliphatic orthoformic esters by the direct addition of hydrogen cyanide to a monohydric alcohol containing hydrogen chloride under anhydrous conditions in a one pot reaction. Yields in the 50% range were disclosed without reference to purity employing methanol and ethanol, although other primary and secondary alcohols were disclosed. It is also known form US-A-4785012 that imino ether hydrochlorides of the general structure (III) below can be obtained in crystalline form by the reaction of HCN, HCl and isopropanol in ether. However, the low temperature reaction (0°C) disclosed is slow (four days) and is of little commercial significance. From GB-A-1128963 it is also known that the reaction of a nitrile with HCl and alcohol will yield ortho esters and that secondary alcohols can be employed. US 2527494 discloses a method of producing orthoformic esters comprising reacting formiminoester hydrohalide with a monohydroxy unsubstituted alcohol in the presence of an inert liquid hydrocarbon non-solvent for ammonium halides. Summary of the Invention The present invention provides the processes of claims 1 and 2 of this specification. The remaining claims specify preferred but optional process steps. Detailed Description of the Invention The present invention relates to the preparation of orthoformic acid alkyl esters of the structure (I) (described above, hereinafter "ortho esters I"). The initial step in the preparation of the ortho esters I is the preparation of a reaction mixture containing an imino ether hydrochloride of the structure (III) (described above, hereinafter "imino ether III"). The imino ether III is prepared by anhydrously reacting hydrogen cyanide with hydrogen chloride and a secondary alkanol of the structure (IV) wherein R 1 and R 2 correspond to R 1 and R 2 of the ortho esters I. As used herein, the term "alkyl moiety" or "alkyl group" used in reference to R 1 and R 2 means any alkyl group of 1-3 carbon atoms and includes, without limitation, saturated, unsaturated, branched, unbranched and cyclic alkyl groups. The reaction must be carried out under anhydrous conditions. This requirement will be met if the overall content of water in the reaction mixture is lower than the preferable limit of about 1,000 ppm. The reaction mixture should remain anhydrous throughout the preparation of the ortho esters I. In the preparation of the imino ether III, the reactants are added approximately simultaneously to the reaction vessel. Approximately equimolar concentrations of the reactants are added to the reaction vessel, with a slight excess of the secondary alkanol and hydrogen chloride being preferred. The preferable molar ratio of hydrogen cyanide to secondary alkanol to hydrogen chloride is 1:1.05:1.10. The addition of the reactants is carried out at a temperature within the range of 0°C to 30°C. A continuous addition over a 10 to 12 hour period is preferred. Once the addition is completed, it is preferable to stir the reaction mixture for about 6 to 8 hours. The reaction mixture is maintained at a temperature within the range of 0°C to 30°C. Following this process, it is possible to obtain an almost quantitative yield of the imino ether III. Prior to the addition of the reactants, the reaction vessel should be charged with an organic solvent. A preferred solvent is any solvent which is essentially anhydrous, does not participate in the reaction and does not dissolve ammonium chloride formed as a by-product. Appropriate solvents include, but are not limited to, n-hexane, cyclohexane, heptane, carbon tetrachloride, decalin, petroleum ether, 2,2,4-trimethylpentane, benzene, ethylbenzene, mesitylene, toluene, and xylene, or mixtures thereof. The amount of solvent is preferably at least equal to the molar amount of hydrogen cyanide to be added and, more preferably, more than twice the total molar amount of hydrogen cyanide and secondary alkanol to be added. If the amount of solvent is too low, the slurry which develops during the formation of the imino ether III will not stir effectively. As stated, the synthesis of the imino ether III is carried out at a temperature within the range of 0°C to 30°C. Above 30°C, the imino ether III should form but it may subsequently decompose, thereby decreasing the ultimate yield of the ortho ester I prepared thereafter. Below 0°C, the imino ether III can form but the reaction time may become prohibitively long in duration. A lengthy reaction time would be an obvious detriment in industrial applications. The optimum temperature depends in part on the type of solvent used. With a nonpolar solvent, such as carbon tetrachloride, a temperature within the range of 10°C to 20°C is preferred. With a polar solvent, such as chloroform, a temperature within the range of 10°C to 30°C is preferred. From the imino ether III, the ortho ester I can be prepared. Separation or purification of the imino ether III, however, is not necessary. Concentration of the reaction mixture under reduced pressure can be done if desired. To prepare the ortho ester I, the imino ether III is subjected to alcoholysis through the addition of more secondary alkanol to the reaction mixture. Prior to or simultaneously with the addition of the secondary alkanol to prepare the ortho ester I, the pH of the reaction mixture is adjusted to within the range of 3 to 5 and, more preferably, to within the range of 4 to 5. The base employed to adjust the pH should be any basic substance which does not give rise to water. Such basic substances include metallic potassium, metallic sodium, potassium alcoholate, sodium alcoholate, ammonia, methylamine, trimethylamine, ethylenediamine, and diethylenediamine. The preferred base is ammonia gas. If an alcoholate is used, it should be based on the same alcohol as the secondary alcohol employed in the reaction. If the alcoholysis is carried out outside the pH range of 3 to 5, the yield of ortho ester I may decrease. If the pH falls below 3, the ortho ester I, which is sensitive to acid, can decompose. If the pH is greater than 5, the imino ether III can become unstable and decompose, thereby lowering the yield of ortho ester I. The alcoholysis is carried out at a temperature within the range of 30°C to 50°C, preferably for a period of about 6 to 12 hours. Although the imino ether III can be unstable above 30°C, in the presence of alcohol, the imino ether III can react to form the ortho ester I in the range of 30°C to 50°C. More preferred is a temperature within the range of 30°C to 40°C. Below 30°C, the alcoholysis may proceed at a relatively slow rate. Above 50°C, the reaction can still proceed but the imino ether III can begin to decompose, thereby reducing the yield of ortho ester I. The amount of secondary alkanol to be added to prepare the ortho ester I is at least twice the molar amount of the imino ether III (assuming a quantitative yield). That is, The amount of secondary alkanol to be added is at least twice the molar amount of hydrogen cyanide initially added. Secondary alkanols of the structure (IV) can be used in the process of the invention. Such alkanols include 2-propanol, 2-butanol, 3-methyl-2-pentanol, 2-pentanol, 3-pentanol and 3-heptanol and the like. It is preferred to carry out the preparation of the ortho esters I under vacuum to minimize the potential loss of hydrogen chloride and hydrogen cyanide from the system. After the preparation of the ortho ester I, the reaction product can be separated from the reaction mixture, as is further explained below. The reaction mixture should first be cooled to a temperature of about 25°C, and preferably to below 10°C, and filtered to remove the ammonium chloride which forms as a by-product. To the filtrate, a base should be added to raise the pH to a level greater than about 7 and preferably within the range of about 8 to 10. Basic substances which can be used to raise the pH include alkali metals, such as potassium and sodium, alcoholates, methylamine, ethylenediamine, diethylenediamine and aqueous caustic solution. If aqueous caustic solution is used, the organic phase should be separated prior to separation of the ortho ester I to minimize the potential decomposition of either product. The ortho ester I can then be obtained by distillation. When the secondary alkanol is present in the reaction mixture, the ortho ester I remains stable even in the presence of ammonium chloride. If the secondary alkanol is a low-boiling alcohol, and evaporates during the filtration of the ammonium chloride, the ortho ester I can become unstable. Thus, it is preferable to carry out the filtration under conditions which would inhibit the removal of the secondary alkanol. For example, carrying out the filtration at a relatively cool temperature should be effective. The following examples further illustrate the process of the invention. Example 1 Preparation of 1,1,1-Triisopropylorthoformate A one liter, 5 necked flask was equipped with an overhead stirrer, thermometer, addition funnel, gas inlet tube, and vacuum outlet through a manostat. Anhydrous toluene was charged into the flask and the system was evacuated with a water aspirator to about 67728-74501 Pa (20-22 inches Hg). The entire reaction was carried out under vacuum in order to minimize the loss of hydrogen chloride and hydrogen cyanide from the system. The addition funnel was connected to a mixture of hydrogen cyanide (27.0 g) and 2-propanol (63.0 g). This mixture was named the "mixed feed". The toluene in the flask was cooled to about 10-15°C, and hydrogen chloride was bubbled through at a rate so as to maintain the system under vacuum. When about 10% of the hydrogen chloride was added, addition of the mixed feed began. The remainder of the hydrogen chloride and mixed feed was added, and reaction mixture stirred, at a rate sufficient to maintain the temperature of the mixture within the range of about 10°C to 20°C. The addition was done over a four hour period. The imino ether hydrochloride salt crystallized out of the reaction mixture. The temperature was maintained below 20°C to prevent decomposition of the imino ether hydrochloride. After the addition, the reaction was stirred for about 8 hours at 20-25°C. Gaseous ammonia was then added to adjust the pH to 4-5 and neutralize the excess hydrogen chloride, during which time the temperature was maintained around 20°C. 2-propanol (150.0 g) was then added in one lot. The temperature of the mixture was raised and maintained at 30-35°C for about 10 hours. Ammonium chloride was filtered off, and the organic phase was washed with a 25% aqueous caustic solution (50 ml). The organic phase was separated from the aqueous phase and distilled under reduced pressure 1333 Pa (10 mm Hg). 135 g (70% yield) of 1,1,1-triisopropylorthoformate was obtained, with a purity of at least 98% as determined by gas chromatographic assay. Example 2 Preparation of Tri(2-pentyl)orthoformate A one liter, 5-necked flask is equipped as in example 1. Anhydrous toluene (2 mole equivalents) is charged into the flask and the system is evacuated with a water aspirator to about 67728-74501 Pa (20-22 inches Hg). The entire reaction is carried out under vacuum. The addition funnel is charged with a mixture of hydrogen cyanide (27.0 g) and 2-pentanol (92.4 g). This mixture is the "mixed feed". The toluene in the flask is stirred and cooled to about 10-15°C, and hydrogen chloride is bubbled through at a rate so as to maintain the system under vacuum. When about 10% of the hydrogen chloride is added, addition of the mixed feed is commenced. The remainder of the hydrogen chloride and mixed feed is added, and the reaction mixture stirred, at a rate sufficient to maintain the temperature of the reaction mixture within 10-20°C. The addition is done over a four hour period. The imino ether hydrochloride salt crystallizes out of the reaction mixture. The temperature is maintained below 20°C to prevent decomposition of the imino ether hydrochloride. After the addition, the reaction is stirred for about 20 hours at 20-25°C. The reaction mixture is cooled to 15°C and gaseous ammonia is added to adjust the pH to 4-5 and neutralize the excess hydrogen chloride. 2-Pentanol (220.0 g) is then added in one lot. The temperature of the reaction mixture is raised and maintained at 40-50°C for about 12 hours. Ammonium chloride is filtered off, and the organic phase is washed with a 25% aqueous caustic solution (50ml). The organic phase is separated from the aqueous phase and distilled under reduced pressure 667 Pa (5 mm Hg) to yield tri(2-pentyl)orthoformate. Example 3 Preparation of Tri(3-pentyl)orthoformate A one liter, 5-necked flask is equipped as in example 1. Anhydrous toluene (2 mole equivalents) is charged into the flask and the system is evacuated with a water aspirator to about 67728-74501 Pa (20-22 inches Hg). The entire reaction is carried out under vacuum. The addition funnel is charged with a mixture of hydrogen cyanide (27.0 g) and 3-pentanol (92.4 g). This mixture is the "mixed feed". The toluene in the flask is stirred and cooled to about 10-15°C, and hydrogen chloride is bubbled through at a rate so as to maintain the system under vacuum. When about 10% of the hydrogen chloride is added, addition of the mixed feed is commenced. The remainder of the hydrogen chloride and mixed feed is added, and the reaction mixture stirred, at a rate sufficient to maintain the temperature of the reaction mixture within 10-20°C. The addition is done over a four hour period. The imino ether hydrochloride salt crystallizes out of the reaction mixture. The temperature is maintained below 20°C to prevent decomposition of the imino ether hydrochloride. After the addition, the reaction is stirred for about 20 hours at 20-25°C. The reaction mixture is cooled to 15°C and gaseous ammonia is added to adjust the pH to 4-5 and neutralize the excess hydrogen chloride. 3-Pentanol (220.0 g) is then added in one lot. The temperature of the reaction mixture is raised and maintained at 50-60°C for about 12 hours. Ammonium chloride is filtered off, and the organic phase is washed with a 25% aqueous caustic solution (50ml). The organic phase is separated from the aqueous phase and distilled under reduced pressure 667 Pa (5 mm Hg) to yield tri(3-pentyl)orthoformate.

Claims: A method of preparing an imino ether hydrochloride of the structure (III) wherein R 1 and R 2 are different or the same alkyl moieties having from 1 to 3 carbon atoms, the method comprising (a) adding approximately equimolar quantities of the reactants hydrogen cyanide, hydrogen chloride and a secondary alkanol of the structure (IV), wherein R 1 and R 2 correspond to R 1 and R 2 of structure (III), approximately simultaneously to an anhydrous organic solvent, at a temperature within the range of 0°C to 30°C, (b) continuing to add the reactants to the organic solvent while reacting said hydrogen cyanide, hydrogen chloride and secondary alkanol of the structure (IV), while maintaining the temperature within the range of 0°C to 30°C, to form the imino ether hydrochloride of the structure (III). A continuous method of preparing an ortho ester of the structure (I) wherein R 1 and R 2 are different or the same alkyl moiety, said alkyl moiety having from 1 to 3 carbon atoms, the method comprising preparing an imino ether hydrochloride of the structure (III) according to the process of claim 1, followed by: (c) adjusting the pH to within the range of 3 to 5, and (d) anhydrously reacting the imino ether hydrochloride, at a temperature within the range of 30°C to 50°C, with additional secondary alkanol of the structure (IV) by adding the additional secondary alkanol in a molar amount equal to at least twice the molar amount of hydrogen cyanide used to make the imino ether hydrochloride. A method according to claim 1 or claim 2, wherein the hydrogen cyanide, hydrogen chloride and secondary alkanol are reacted in a molar ratio of the hydrogen cyanide to secondary alkanol to hydrogen chloride of 1 : 1.05 : 1.10. A method according to any preceding claim, wherein the organic solvent is inert in the reaction mixture and does not dissolve ammonium chloride. A method according to any preceding claim, wherein the organic solvent comprises n-hexane, cyclohexane, heptane, carbon tetrachloride, decalin, petroleum ether, 2,2,4-trimethylpentane, benzene, ethylbenzene, mesitylene, toluene, xylene or chloroform. A method according to claim 5, wherein the organic solvent comprises toluene. A method according to any preceding claim, wherein the hydrogen cyanide, hydrogen chloride and secondary alkanol of the structure (IV) are reacted in a polar solvent at a temperature within the range of 10°C to 30°C, or in a non-polar solvent at a temperature within the range of 10°C to 20°C, to form the imino ether hydrochloride. A method according to any preceding claim, and further comprising, after preparing the imino ether hydrochloride, adjusting the pH of the solvent containing the imino ether hydrochloride to 4 to 5. A method according to claim 8, wherein the pH is adjusted prior to reacting the additional secondary alkanol. A method according to any preceding claim, wherein the secondary alkanol is selected from 2-propanol, 2-butanol, 3-methyl-2-pentanol, 2-pentanol, 3-pentanol and 3-heptanol. A method as claimed in any preceding claim, wherein about 10% of the hydrogen chloride is added to the anhydrous organic solvent prior to addition of the hydrogen cyanide and the secondary alkanol. A method as claimed in any preceding claim, wherein the hydrogen cyanide and the secondary alkanol are added to the anhydous organic solvent as a mixed feed.

Inventors: BITLER GEORGES W, COLVIN DAVID, JOHN THOMAS V, SUBRAMANIAM CHITOOR S, BITLER, GEORGES W, COLVIN, DAVID, JOHN, THOMAS V, SUBRAMANIAM, CHITOOR S
Applicants: HULS AMERICA INC, HUELS AMERICA INC.
Application year: 1994
Main classification: C07C41
IPC codes: C07C41, C07C257, C07C43